Method and apparatus for filtering digital television signals

ABSTRACT

A system for filtering digital television signals is provided. The system comprises a generator for providing a first data sequence to a private data packetizer, and a transmitter for transmitting the packetized first data sequence in a data channel of a digital television signal. The system further includes a receiver for receiving the digital television signal and recovering the first data sequence. The receiver includes a channel estimator for providing an estimate of channel characteristics, such as estimated channel impulse estimate and estimated noise variance. The receiver further includes an adaptive equalizer filter having an input for receiving the digital television signal and an input for receiving adaptive filter coefficients. The receiver further includes a coefficient processor for calculating adaptive filter coefficients based on the channel estimate, and providing the adaptive filter coefficients to the adaptive equalizer filter. The digital television signal is thus filtered to remove undesired channel effects.

This is a continuation of application Ser. No. 10/728,934 filed Dec. 5,2003, abandoned, which is a continuation of application Ser. No.10/247,455 filed Sep. 19, 2002, abandoned, which is a reissue ofapplication Ser. No. 09/206,409, filed Dec. 7, 1998, Pat. No. 6,122,015.

BACKGROUND OF THE INVENTION

This invention relates in digital television and, more particularly, tomethods and apparatus for filtering digital television signals to removemultipath and other undesirable effects upon a digital television signalas the signal propagates through a channel.

Digital television is an emerging technology that is the subject of muchresearch both in the United States and Japan. Because of the potentialadvantages of digital television and the many technical problemsassociated therewith, research into improved systems and methods fortransmitting and receiving digital television signals is increasing.

One of the most important prevalent problems associated with digitaltelevision signals is the problem of multipath effects. The termmultipath, as used herein, refers to the propagation of electromagneticwaves along various paths from the digital television transmitter to thedigital television receiver. Multipath effects may arise from fixedstructures, such as building walls, acting as reflectors in thetransmission channel. Moving objects, such as airplanes, may also causea multipath condition. Even microreflections in cabling can causemultipath conditions. These structures can cause transmission of thetelevision signal to occur along more than one path from the transmitterto the receiver. As a result, the same signal may be received more thanonce, and at different times by a single, or multiple, receivers. Theresult of multipath effects in analog television is to create “ghosts”in the displayed television image. In digital television, the effects ofmultipath can include moderate to severe degradation in the displayed TVpicture and sound.

Various methods and systems have been designed to address the problem ofmultipath. See, for example, P. T. Marhiopoulos and M. Sablatash,“Design of a Ghost Canceling

Reference Signal for Television Systems in North America,” Proceedingsof Canadian Conference on Electrical and Computer Engineering,Vancouver, BC, Canada, Sep. 14-17, 1993, pp. 660-663.

The statistics of multipath ghosts have been studied and compiled by,among others, the BTA (Japan's Broadcasting Technology Association). TheBTA, and other concerns, designed a “ghost canceling reference” (GCR)transmitted signal to mitigate these multipath effects. The BTA GCR wasfound to be less than satisfactory in some cases. While homes withoutdoor antennas displayed non-varying (stationary) ghosting conditionswhich could be largely corrected, those homes with indoor antennasexperienced changing (dynamic) ghosts. These ghosting conditions weremore prevalent where people were moving about the room or other movingobjects were in the signal path. The BTA ghost canceller generally wasnot able to adequately compensate for these conditions. Therefore, aneed remains for a system and method for filtering out, or removing,multipath components from digital television signals, and especially forsystems and methods for filtering multipath components from a digitaltelevision signal when the multipath component arises from movingobjects and dynamic conditions in a transmission channel.

BRIEF SUMMARY OF THE INVENTION

In an exemplary embodiment of the invention a system for filteringdigital television signals comprises a generator for providing a firstdata sequence to a private data packetizer, and a transmitter fortransmitting the packetized first data sequence in a data channel of adigital television signal. The system further includes a receiver forreceiving the digital television signal and recovering the first datasequence. The receiver includes a channel estimator for providing anestimate of channel characteristics such as estimated channel impulseresponse and estimated noise variance. The receiver further includes anadaptive equalizer filter having an input for receiving the digitaltelevision signal and an input for receiving adaptive filtercoefficients. The receiver further includes a coefficient processor forcalculating adaptive filter coefficients and providing the adaptivefilter coefficients to the adaptive equalizer filter. The equalizerfilter is in communication with the output of the comparing circuit suchthat filter coefficients of the adaptive filter are adjusted accordingto the estimate of the impulse response of the data channel. In oneembodiment of the present invention, the television transmission iscoded according to a Motion Picture Experts Group (MPEG-2) standard.

A method of filtering a digital television transmission comprises thesteps of generating a first data sequence at a transmitter andperiodically inserting the first data sequence into a digital televisionbit stream to be transmitted. The method further comprises the steps oftransmitting the digital television bit stream through a channel to areceiver, receiving the digital television bit stream and extracting thefirst data sequence from the digital television bit stream. Theextracted first data sequence includes channel induced noise. The methodfurther includes the steps of comparing the extracted first datasequence, including channel induced noise, to a second data sequence.The second data sequence is locally generated, that is, the second datasequence is generated at the receiver and does not include channelinduced noise. However, in one embodiment of the present invention thesecond data sequence contains the same data as the first data sequence.The method further includes a step of provide a channel estimate basedon the comparison step. The method further includes the steps ofapplying the received television bit stream to an adaptive filter andadaptively adjusting filter coefficients of the adaptive filteraccording to the channel estimate such that undesirable channel effects,such as noise, upon said received television bit stream are filteredfrom said received television bit stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a block diagram of a digital television transmitter accordingto an embodiment of the present invention;

FIG. 1A is a block diagram of a digital television receiver including afilter according to the present invention;

FIG. 2 is a block diagram of an embodiment of a channel estimatoraccording to the invention;

FIG. 3 is a block diagram of one embodiment of an equalizer filteraccording to the present invention;

FIG. 4 is a flow diagram of a method for generating equalizercoefficients according to one embodiment of the present invention;

FIG. 5 is a block diagram of a decrypter according to one embodiment ofthe present invention; and

FIG. 6 is a timing diagram showing a rolling frame training sequenceaccording to the fourth aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION Transmitter

FIG. 1 is a block diagram of one embodiment of a digital televisiontransmitter 10 adapted according to the present invention. Digitaltelevision transmitter 10 includes circuitry typically used andgenerally known in the art of digital television to packetize and encodeaudio and video signals for transmission to a digital televisionreceiver. Typical circuits include video encoder/packetizer 12, audioencoder/packetizer 14 and private data encoder/packetizer 16. Privatedata encoder/packetizer 16 is used to encode and packetize what is knownin the art as a private data stream. Typical uses for a private datastream include carrying parity bits for other data packets, allowing forincreased error detection and correction. The private data stream mayalso be used to carry measured distortion. Measured distortion isdistortion purposely introduced as part of a pay for quality service.

The present invention relies upon use of the private data stream of apacketized digital television signal. According to the presentinvention, transmitter 10 includes a data sequence generator 20. Datasequence generator 20 generates a predetermined sequence of digitalbits. Any sequence of bits can be selected for generation by first datasequence generator 20. The particular sequence selected is not importantas long as the sequence is a known and repeatable bit pattern suitablefor packetization. The data sequence from first data sequence generator20 is provided to private encoder/packetizer 16 where it is encoded andpacketized in the same manner as other private data such as parity bits.

The packetized data sequence is then combined by multiplexer 22 withpacketized audio and video signals in accordance with a digitalformatting standard to provide digital television stream 25. In oneembodiment of the present invention the MPEG-2 (Motion Picture ExpertGroup) coding standard is employed. In alternative embodiments of thepresent invention the digital television signal is coded generally inaccordance with the ATSC (Advanced Television Standards Committee)standard. The ATSC standard, which includes the MPEG format, allows each19.3 Mbps of information to be time divided into video, audio, andprivate data channels. The information is transmitted in packets of 188bytes, and each packet begins with a packet identifier (indicating whichdata stream it belongs to, for example, voice stream number three orvideo stream number one, etc.). Accordingly the packet containing thedata sequence generated by first data sequence generator 20 isidentified as belonging to a “private” data stream. Digital televisionstream 25 is transmitted, or broadcast, as a digital television signalthrough channel 600 to a digital television receiver. It will berecognized by those skilled in the art that the invention is not limitedto MPEG signal formats. In fact, the invention could be utilized withany digital television signal carrying information in packets.

Receiver

FIG. 1A is a block diagram of a digital television receiver 50 adaptedaccording to the present invention. A television receiver designedwithout knowledge of the novel use of the private data channel wouldsimply discard data sequence information. Television receivers designedin accordance with the present invention, effectively utilize the datasequence information as described below to improve the quality of thereceived signal.

The digital television signal, including the packetized video, audio andprivate data sequences, is received and pre-processed by receiver frontend 55 to demodulate the signal and to recover digital television stream25. Recovered digital television stream 25a is essentially the samesignal as digital television stream 25, but may have been degraded,corrupted, or otherwise affected by particular propagationcharacteristics of channel 600.

Private data stream 101 is recovered from digital television stream 25aby demultiplexer 60. As previously described herein, the information onthe private data stream comprises a known data sequence. The receivedknown data sequence is indicated in FIG. 1A as first data sequence 101.First data sequence 101 is provided to channel estimator 100. Alsoprovided to channel estimator 100 is local data sequence 103. Local datasequence 103 is the same data sequence as first data sequence 101,except local data sequence 103 has not been affected by thecharacteristics of channel 600. Channel estimator 100 estimates thepropagation characteristics of channel 600 by comparing local datasequence 103 to first data sequence 101. In other words, channelestimator 100 provides a channel estimate based on comparison of datasequence 102 and 103. In one embodiment of the present invention, thechannel estimate comprises estimated channel impulse response signal 115and estimated noise variance signal 120. Estimated channel impulseresponse signal 115 and estimated noise variance signal 120 are providedto coefficient processor 500.

Coefficient processor 500 determines filter coefficients, which, whenapplied to equalizer 300, causes equalizer 300 to undo the effects ofchannel 600 on recovered digital television stream 25a. Thus, multipathcomponents and other distortions in the signal are filtered from thedigital television signal. The filtered audio and video streams may thenbe recovered from the digital television signal and processed inaccordance with methods and apparatus known in the art.

According to one embodiment of the present invention (not shown), aLeast Mean Squares (LMS) algorithm is employed to obtain filtercoefficients to be applied to equalizer 300. However, this technique hasa drawback in that the convergence time of the least mean square (LMS)algorithm is inversely proportional to the smallest eigenvalue of theautocorrelation matrix of the received sequence (input to equalizer). Onchannels with severe multipath and in-band nulls, this smallesteigenvalue becomes very close to zero which could slow down the LMSequalizer convergence.

An embodiment of the invention that avoids this autocorrelation matrixproblem is illustrated in FIG. 1A. The embodiment of FIG. 1A includes achannel estimator 100. Channel estimator 100 obtains a channel estimateand provides the channel estimate to coefficient processor 500.Coefficient processor 500 then uses the channel estimate to computeequalizer coefficients to be applied to the taps of adaptive equalizer300. The term channel estimate, as used herein means the channelcharacteristics as represented by the impulse response signal 115 andestimated noise variance signal 120. Channel impulse response signal 115is estimated based on comparison of data sequence 101 (embedded in eachtransmitted block of the digital television bit stream 25), and locallygenerated data sequence 103. The channel estimate is then used, togetherwith an estimate of the channel signal-to-noise ratio (SNR), i.e.,estimated noise variance signal 120, to compute the optimum coefficientsof adaptive equalizer 300. Received digital television stream 25a isapplied to the input of equalizer filter 300. The response of equalizer300 to digital television stream 25a, as determined by equalizercoefficients 130 and 140, is such that undesirable channel effects ondigital television stream 25a are filtered from digital televisionstream 25a.

Channel Estimator

One embodiment of channel estimator 100 is depicted in FIG. 2. Aspreviously described, a known data sequence 101 is transmitted over achannel 600. The received data sequence signal 101 has passed throughchannel 600 and has likely been corrupted by noise. Data sequence 101 isapplied to first Fast Fourier Transformer (FFT) 102 which computes theFFT of noisy and distorted received data sequence signal 101. Locallygenerated data sequence 103 is generated by second data sequencegenerator 148 (best ilustrated in FIG. 1A) and applied to second FastFourier Transformer 104. Second FFT 104 computes the FFT of second datasequence 103. Divider 105 divides the FFT of the received signal by theFFT of the locally-generated data sequence. The output of divider 105 isan estimate of the channel frequency response.

The channel impulse response signal 115 is computed by Inverse FFT(IFFT) 106 which computes the inverse FFT of the computed channelfrequency response output from divider 105. The resulting time-domainresponse is provided to window circuit 107 to obtain an estimatedchannel response {dot over (h)}=[h₀ h₁ . . . h_(ν)]. The startinglocation and width of window 107 is determined by requiring that thewindowed impulse response contains most of the energy (e.g., 99% orhigher) of the un-windowed channel impulse response..

An estimate of the noise variance, and hence the channel signal-to-noiseratio (SNR) is determined by computing the average energy of the channelestimation error sequence. The channel estimation error sequence isequal to the difference output of subtractor 109. Subtractor 109computes the difference between the actual received sequence and theestimated received sequence (formed by convolving in convolutionfunction 108 the locally-generated training signal on channel 103 withthe estimated windowed channel impulse response from the window function107) and the received signal on channel 101. The average energyestimation is then computed by function 110 as${\frac{1}{L}{\sum\limits_{i = 1}^{L}\quad{\bullet }^{2}}},$where L represents the length of the received sequence, and |●|represents the absolute value of the received sequence function.

Once the channel impulse response and channel SNR estimates areavailable, they are provided to coefficient processor 500 and used tocompute the optimum equalizer coefficients for equalizer 300.

Equalizer

The equalizer structure employed in one embodiment of the invention isthe minimum mean square error decision-feedback equalizer (MMSE-DFE)shown in FIG. 3. This equalizer structure consists of twofinite-impulse-response (FIR) filters 201, and 202. The first FIR filteris a feed forward filter 201 (denoted by w), and the second FIR filteris a feedback filter 202 (denoted by b). FIR filter 201 receives digitaltelevision stream 25a at an input and outputs a filtered signal tosummer 204. The output of summer 204 is provided to a decision device205. The output of decision device 205 is estimated symbols denoted{circumflex over (x)}_(k-Δ) This output is also fed back to FIR filter202, the filtered output of which constitutes the second input to summer204. It is this output that is subtracted from the output of FIR filter201. The coefficient settings of the two FIR filters 201 and 202 areoptimized to minimize the mean square value (or equivalently averageenergy) of the error sequence (which is equal to the difference betweenthis input and output of the decision device 205 in FIG. 3). Previouslydetected-symbols, denoted in FIG. 3 by {circumflex over (x)}_(k-Δ), arefed back and filtered to remove their interfering effect of current andfuture symbols yet to be detected. The coefficients for both FIR filters201 and 202 are provided by coefficient processor 500.

Coefficient Processor

In one embodiment of the present invention coefficient processor 500computes the equalizer coefficients from impulse response signal 115 ina non-iterative (i.e., one shot) computation that has a closed form andis coded on a programmable digital signal processor (DSP) chip. In oneembodiment of the present invention, computing the optimum equalizercoefficients from the channel impulse response estimate is accomplishedby inverting a correlation matrix whose size is equal to the total (feedforward and feedback) number of equalizer taps according to methods wellknown to those of ordinary skill in the art of signal processing. Otherequalizer computation algorithms suitable for use in conjunction withthe present invention are known to those of ordinary skill in the art.In one embodiment of the present invention the equalizer computationalgorithm is implemented on a commercially available programmabledigital signal processor. In another embodiment of the presentinvention, the equalizer computations are implemented on an ASIC. Aswill be readily apparent to those of ordinary skill in the art, otherintegrated circuits or processor means may be employed to executealgorithms for computing equalizer coefficients.

A flow chart of the steps of a method and algorithm of the presentinvention implemented by coefficient processor 500 to compute theoptimum filter coefficients of equalizer 300 is shown in FIG. 4. Theestimated channel impulse response and estimated noise variance are usedto construct the matrix R in function step 301, where$R = {{\frac{1}{SNR}I_{N + v}} + {H*{H.}}}$This matrix is then factorized in step 302 into the product of alower-triangular matrix L, ${L = \begin{bmatrix}1 & 0 & 0 & 0 \\x & 1 & 0 & 0 \\x & x & 1 & 0 \\x & x & x & 1\end{bmatrix}},$a diagonal matrix D, ${D = \begin{bmatrix}d_{1} & 0 & 0 & \cdots & 0 \\0 & d_{2} & 0 & \cdots & 0 \\0 & 0 & ⋰ & \cdots & 0 \\\vdots & \vdots & \vdots & ⋰ & \vdots \\0 & 0 & 0 & \cdots & d_{N + v}\end{bmatrix}},{and}$an upper-triangular matrix L*, where * denotes the complex conjugatetranspose operation. This factorization is commonly known as a“triangular” or “Cholesky” factorization in matrix theory.

This triangular factorization contains all the information needed tocompute the optimum MMSE-DFE filter settings and determine the optimumdelay parameter Δ. More specifically, the optimum delay, Δ_(opt), isequal to the index of the largest diagonal element of matrix D infunction block 303. That is, $\Delta_{opt} = {\begin{matrix}{\arg\quad\max\left\{ d_{i} \right\}} \\{1\quad \leq i \leq {N + v}}\end{matrix}.}$

The optimum feedback filter coefficients 140 (designated as Filter) areset to Δ_(opt) ^(th) column of the matrix L in step 304. The optimumfeed forward filter coefficients 130 are obtained in step 305 bymultiplying the channel matrix H by the Δ_(opt) ^(th) column of thecomplex conjugate transpose of the L⁻¹ matrix and dividing the resultingcolumn vector by the scalar d_(Δ) _(opt) , that is ${H = \begin{bmatrix}h_{0} & h_{1} & \cdots & h_{v} & \cdots & \cdots & 0 \\0 & h_{0} & ⋰ & ⋰ & ⋰ & ⋰ & 0 \\0 & 0 & h_{0} & h_{1} & \cdots & h_{v} & 0 \\0 & 0 & \cdots & h_{0} & h_{1} & \cdots & h_{v}\end{bmatrix}},$and e_(Δ) _(opt) is a unit column vector which has a “1” in its _(Δ)_(opt) entry and zeros everywhere else; e.g.,${e\Delta}_{opt} = \left. \begin{bmatrix}0 \\\vdots \\0 \\1 \\0 \\\vdots \\0\end{bmatrix}\leftarrow{\Delta_{opt}.} \right.$

Encryption

According to another embodiment of the invention, the data sequence isencrypted. An encryption feature, of any type generally available andknown, is added to the data sequence so that the known data sequencewill be available only to qualified receivers such as, for example,those subscribers who have paid a periodic of pay-for-view accesscharge. When applied in combination with other embodiments of theinvention, a private data channel packet is received and if it isencrypted, the packet is first decrypted by a receiver. In oneembodiment of the present invention this is accomplished by utilizing akeying variable. The un-encrypted or decrypted data bits of the privatedata channel packet may be used for different purposes, such to provideerror detection for other received packets' data, to provide errorcorrection for other received packets' data, to provide data that couldbe used to remove measured data distortion or distortion that waspurposely introduced as part of a pay for quality service, as well as toprovide a channel estimate for reduction of multipath effects.

Encryption of the data sequence may be added so that it may be availableonly to qualified receivers. Encryption/decryption is accomplished by a“classical” or “one-key” cryptographic system such as the DataEncryption Standard (DES) as defined in the Federal InformationProcessing Standard 46 (1977) and in a mode defined in FederalInformation Processing Standard 81 (1980). Further cryptographicarchitectural information regarding the DES and its modes of operationis contained in the article “Data Encryption Standard,” by Hershey andPomper, and is found in Vol. 5, pp. 227-251 of the Froehlich/KentEncyclopedia of Telecommunications, Marcel Dekker, Inc.

In practice, only digital data to be transmitted is encrypted. As shownin FIG. 5 at the receiver, the received encrypted digital data is inputto a decryptor 401 which outputs decrypted digital data. This output isinput to a digital-to-analog (D/A) converter 402 which generates ananalog waveform from the decrypted digital data.

Dynamic Frame Structure

In one embodiment of the invention, a dynamic or rolling frame/packetstructure is used as depicted in FIG. 6. This structure allows a datasequence and staggered slots to be used for countering multipath and, inthis way, different multipath delays may be easily estimated. Asillustrated in FIG. 6, T stands for Training Sequence Interval and Dstands for Data Sequence Interval. Data sequences are bits, whiletraining sequence intervals may contain bits or segments of a speciallycrafted channel diagnostic waveform. The example shown in FIG. 5 has arepetition length of four frames and inter-training sequence intervalsthat are spaced so that measurements may be easily made over manydifferent time delays. For example, if there were errors that typicallyoccurred between segments spaced by four micro-seconds but not one ortwo or three or five, then it is concluded that there is a strongmultipath component at four microseconds.

The training sequence and staggered slots for countering multipathdepends upon there being a large number of different inter-intervalspacings. By sending known wave-forms (modulated bits) in trainingintervals (i.e., the Ts), it is possible to locate, within an intervalof time, the delays of the strong multipath components. For example, iftwo identical transmitted waveform Ts are separated by four timeintervals and the corresponding received Ts are not identical, anaverage value of their difference can be formed, and this average valuewill reflect the average multipath component at a delay of four timeintervals. In the time domain, let x(t) be a known training sequence andassume that x(t+4) was also transmitted. Assume that there is a strongmultipath component with delay τ+4. Assume y(t) and y(t+4),respectively, are received. The average difference is computed asδ(t)=y(t+4)−y(t), where this average difference removes the effects ofuncorrelated multipath from other intervals. What δ(t) produces is−kx(t−τ) where k is the strengths of the multipath, k<1 and τ⁻4. If theaverage value of δ(t) is not zero, then an estimate of k and τ is solvedor, in the alternative, the knowledge that there is a large multipathcomponent with τ⁻4 is used as ancillary information to aid the secondaspect of the invention of estimation of equalizer directly from thechannel information.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

1. A method of filtering a digital television transmission comprisingthe steps of: generating a plurality of packetized first data sequencesequences distributed over a training interval at a transmitter;transmitting through a channel, a digital television stream includingsaid packetized first data sequence sequences; receiving said digitaltelevision stream at a receiver and recovering said first data sequencesequences from said digital television stream; comparing ones of saidfirst data sequence sequences to a second data sequence, said seconddata sequence being locally generated, to provide a channel estimate;applying said received television bit stream to an adaptive filter;adaptively adjusting filter coefficients of said adaptive filteraccording to said channel estimate such that undesirable channel effectsupon said received television stream are filtered from said receivedtelevision stream, wherein the digital television transmission is adigital television signal and said data sequences are transmitted in aprivate channel of an MPEG (Motion Picture Expert Group) data channel.2. The method of filtering a digital television transmission accordingto claim 1 wherein the digital television transmission is a highdefinition television (HDTV) signal and said data sequences aretransmitted in a private channel of an MPEG (Motion Picture ExpertGroup) data channel.
 3. The method of filtering a digital televisiontransmission according to claim 1 wherein said first data sequence iscorrupted by noise after passage through said channel to said receiver.4. The method of filtering a digital television transmission toaccording to claim 3 wherein said first data sequence corrupted by noiseis used to compute an estimate of channel frequency response.
 5. Themethod of filtering a digital television transmission according to claim4 A method of filtering a digital television transmission comprising thesteps of: generating a packetized first data sequence at a transmitter;transmitting through a channel, a digital television stream includingsaid packetizing first data sequence; receiving said digital televisionstream at a receiver and recovering said first data sequence from firstdata sequence from said digital television stream; comparing said firstdata sequence to a second data sequence, said second data sequence beinglocally generated, to provide a channel estimate; applying said receivedtelevision bit stream to an adaptive filter; adaptively adjusting filtercoefficients of said adaptive filter according to said channel estimatesuch that undesirable channel effects upon said received televisionstream are filtered from said received television stream, wherein saidfirst data sequence is corrupted by noise after passage through saidchannel to said receiver and is used to compute an estimate of channelfrequency response, and wherein the step of comparing comprises thesteps of: computing a Fast Fourier Transform (FFT) of said first datasequence corrupted by noise; computing a FFT of said second datasequence; and dividing the FFT of said first data sequence by the FFT ofsaid second data sequence to provide said estimate of channel frequencyresponse.
 6. The method of filtering a digital television transmissionaccording to claim 5 further comprising the step of determining saidchannel impulse frequency response using a quotient from said step ofdividing.
 7. The method of filtering a digital television transmissionaccording to claim 6 wherein the step of determining said channelimpulse frequency response comprises the step of windowing an InverseFFT (IFFT) of said quotient of the dividing step.
 8. The method offiltering a digital television transmission according to claim 7 furthercomprising the step of estimating noise variance by computing averageenergy of channel estimation error sequence as a function of saidwindowed IFFT, said first data sequence corrupted by noise and saidsecond data sequence.
 9. The method of filtering a digital televisiontransmission according to claim 8 wherein said step of estimating noisevariance comprises the steps of: convolving said windowed IFFT with saidsecond data sequence to generate an estimated noiseless output;subtracting said estimated noiseless output from said first datasequence corrupted by noise to generate a difference signal; andcomputing an average energy estimation from said difference signal. 10.The method of filtering a digital television transmission according toclaim 9 wherein said channel impulse frequency response and the estimateof noise variance are used to computer optimum equalizer coefficientsfor the step of adaptively adjusting filter coefficients.
 11. The methodof filtering a digital television transmission according to claim 1wherein said first transmitted data sequence is encrypted.
 12. Themethod of filtering a digital television transmission according to claim1 A method of filtering a digital television transmission comprising thesteps of: generating a packetized first data sequence at a transmitter;transmitting through a channel, a digital television stream includingsaid packetizing first data sequence; receiving said digital televisionstream at a receiver and recovering said first data sequence from firstdata sequence from said digital television sequence; comparing saidfirst data sequence to a second data sequence, said second data sequencebeing locally generated, to provide a channel estimate; applying saidreceived television bit stream to an adaptive filter; adaptivelyadjusting filter coefficients of said adaptive filter according to saidchannel estimate such that undesirable channel effects upon saidreceived television stream are filtered from said received televisionstream, wherein said first data sequence is transmitted in a dynamic orrolling frame/packet structure.
 13. A system An apparatus for filteringa digital television signal comprising: a generator for generating firstdata sequences at a transmitter and the transmitter for broadcastingsaid digital television signal including said first data sequences in abroadcast channel; a receiver for receiving the digital televisionsignal, said receiver including: a channel estimator for comparing saidfirst data sequences to second data sequences, said second datasequences being locally generated, and for providing an estimate of theimpulse response of said channel at an output of said channel estimator;and an adaptive equalizer filter including an input for receiving saiddigital television signal, and filter taps in communication with saidoutput of said channel estimator such that filter coefficients of saidadaptive filter are adjusted according to said estimate of said impulseresponse of said channel, wherein the digital television signalcomprises a high definition television (HDTV) signal and said first datasequences are transmitted in a private data stream of an MPEG (MotionPicture Expert Group) channel.
 14. The apparatus for filtering a digitaltelevision signal according to claim 13 wherein the digital televisionsignal comprises a high definition television (HDTV) signal and saidfirst data sequences are transmitted in a private data stream of an MPEG(Motion Picture Expert Group) channel.
 15. The apparatus for filtering adigital television signal according to claim 13 wherein said first datasequences are corrupted by noise after passage through the channel tothe receiver.
 16. The apparatus for filtering a digital televisionsignal according to claim 15 wherein said first data sequences corruptedby noise are used to compute an estimate of the frequency response ofsaid channel.
 17. The apparatus for filtering a digital televisiontransmission according to claim 16 An apparatus for filtering a digitaltelevision signal comprising: a generator for generating first datasequences at a transmitter and the transmitter for broadcasting saiddigital television signal including said first data sequences in abroadcast channel; a receiver for receiving the digital televisionsignal, said receiving including; a channel estimator for comparing saidfirst data sequences to second data sequences, said second datasequences being locally generated, and for providing an estimate of theimpulse response of said channel at an output of said channel estimator;and an adaptive equalizer filter including an input for receiving saiddigital television signal, and filter taps in communication with saidoutput of said channel estimator such that filter coefficients of saidadaptive filter are adjusted according to said estimate of said impulseresponse of said channel, wherein the digital television signalcomprises a high definition television (HDTV) signal and said first datasequences are corrupted by noise after passage through the channel tothe receiver, said first data sequences corrupted by noise being used tocompute an estimate of the frequency response of said channel, whereinsaid channel estimator comprises: a first Fast Fourier Transform (FFT)processor for computing a FFT of said first data sequences corrupted bynoise; a generator for generating the second data sequences at thereceiver; a second FFT processor for computing a FFT of said second datasequences; and a divider for dividing an output of said first FFTprocessor by an output of said second FFT processor to produce saidestimate of channel frequency response.
 18. The apparatus for filteringa digital television signal according to claim 17 further comprising anestimator for estimating the channel impulse response using a quotientfrom said divider.
 19. The apparatus for filtering said digitaltelevision signal according to claim 18 wherein said estimator includesa processor for windowing an Inverse FFT (IFFT) of the quotient of thedivider.
 20. The apparatus for filtering a digital television signalaccording to claim 19 further comprising a channel estimator configuredto provide an estimate of noise variance by computing average energy ofchannel estimation error sequence as a function of said windowed IFFT,said first data sequences corrupted by noise and said second datasequences.
 21. The apparatus for filtering a digital television signalaccording to claim 20 wherein said noise variance estimator comprises: aconvolver for convolving said windowed IFFT with said second datasequences to generate an estimated noiseless output; a subtractor forsubtracting said estimated noiseless output from said first datasequences corrupted by noise to generate a difference signal; and aprocessor for computing an average energy estimation from saiddifference signal.
 22. The apparatus for filtering a digital televisionsignal according to claim 20 wherein said channel impulse response andsaid estimate of noise variance are used to compute optimum equalizercoefficients for adaptively adjusting filter coefficients.
 23. Theapparatus for filtering a digital television signal according to claim13 wherein said first data sequences are encrypted.
 24. The apparatusfor filtering a digital television signal according to claim 13 Anapparatus for filtering a digital television signal comprising: agenerator for generating first data sequences at a transmitter and thetransmitter for broadcasting said digital television signal includingsaid first data sequences in a broadcast channel; a receiver forreceiving the digital television signal, said receiver including: achannel estimator for comparing said first data sequences to second datasequences, said second data sequences being locally generated, and forproviding an estimate of the impulse response of said channel at anoutput of said channel estimator; and an adaptive equalizer filterincluding an input for receiving said digital television signal, andfilter taps in communication with said output of said channel estimatorsuch that filter coefficients of said adaptive filter are adjustedaccording to said estimate of said impulse response of said channel,wherein said first data sequences are transmitted in a dynamic orrolling frame/packet structure.
 25. A method of processing a digitaltelevision stream including a plurality of first data sequencesdistributed over a training interval transmitted through a channelcomprising the steps of: receiving said digital television stream at areceiver and recovering said first data sequence from said digitaltelevision stream; comparing at least one of said first data sequencesto a second data sequence, said second data sequence being locallygenerated; applying said received television stream to an adaptivefilter; adaptively adjusting filter coefficients of said adaptive filteraccording to said comparing step such that undesirable channel effectsupon said received television stream are filtered from said receivedtelevision stream, wherein the digital television stream is a highdefinition television (HDTV) signal and said first data sequences aretransmitted in a channel of an MPEG (Motion Picture Expert Group) datachannel.
 26. The method of processing a digital television streamaccording to claim 25 wherein said first data sequences are corrupted bynoise after passage through said channel to said receiver.
 27. Themethod of processing a digital television stream to according to claim26 wherein said first data sequences corrupted by noise are used tocompute an estimate of channel frequency response.
 28. The methodprocessing a digital television stream according to claim 25 whereinsaid first data sequence is encrypted.
 29. A method of processing adigital television stream including a packetized first data sequencetransmitted through a channel comprising the steps of: receiving saiddigital television stream at a receiver and recovering said first datasequence from said digital television stream; comparing said first datasequence to a second data sequence, said second data sequence beinglocally generated, to provide a channel estimate; applying said receivedtelevision stream to an adaptive filter; adaptively adjusting filtercoefficients of said adaptive filter according to said channel estimatesuch that undesirable channel effects upon said received televisionstream are filtered from said received television stream, wherein saidfirst data sequence is corrupted by noise after passage through saidchannel to said receiver and is used to compute an estimate of channelfrequency response, wherein the step of comparing comprises the stepsof: computing a Fast Fourier Transform (FFT) of said first data sequencecorrupted by noise; computing a FFT of said second data sequence; anddividing the FFT of said first data sequence by the FIT of said seconddata sequence to provide said estimate of channel frequency response.30. The method of processing a digital television stream according toclaim 29 further comprising the step of determining said channelfrequency response using a quotient from said step of dividing.
 31. Themethod of processing a digital television stream according to claim 30wherein the step of determining said channel frequency responsecomprises the step of windowing an Inverse FFT (IFFT) of said quotientof the dividing step.
 32. The method of processing a digital televisionstream according to claim 31 further comprising the step of estimatingnoise variance by computing average energy of channel estimation errorsequence as a function of said windowed IFFT, said first data sequencecorrupted by noise and said second data sequence.
 33. The method ofprocessing a digital television stream according to claim 32 whereinsaid step of estimating noise variance comprises the steps of:convolving said windowed IFFT with said second data sequence to generatean estimated noiseless output; subtracting said estimated noiselessoutput from said first data sequence corrupted by noise to generate adifference signal; and computing an average energy estimation from saiddifference signal.
 34. The method of processing a digital televisionstream according to claim 33 wherein said channel frequency response andthe estimate of noise variance are used to compute optimum equalizercoefficients for the step of adaptively adjusting filter coefficients.35. A method of processing a digital television stream including apacketized first data sequence transmitted through a channel comprisingthe steps of: receiving said digital television stream at a receiver andrecovering said first data sequence from said digital television stream;comparing said first data sequence to a second data sequence, saidsecond data sequence being locally generated, to provide a channelestimate; applying said received television stream to an adaptivefilter; adaptively adjusting filter coefficients of said adaptive filteraccording to said channel estimate such that undesirable channel effectsupon said received television stream are filtered from said receivedtelevision stream, wherein said first data sequence is transmitted in adynamic or rolling frame/packet structure.
 36. A method of transmittinga digital television stream through a channel comprising the steps of:generating a plurality of packetized first data sequences distributedover a training interval to be compared in a receiver coupled to saidchannel to a second data sequence, said second data sequence beinglocally generated, to adaptively adjust filter coefficients of anadaptive filter such that undesirable channel effects on a televisionstream received from said channel are filtered from said receivedtelevision stream; and transmitting through said channel, a digitaltelevision stream including said packetized first data sequences,wherein said first data sequences are transmitted in a private channelof an MPEG (Motion Picture Expert Group) data channel.
 37. The method oftransmitting a digital television stream according to claim 36 whereinsaid first data sequences are corrupted by noise after passage throughsaid channel to said receiver.
 38. The method of transmitting a digitaltelevision stream to according to claim 37 wherein said first datasequences corrupted by noise are used to compute an estimate of channelfrequency response.
 39. The method of transmitting a digital televisionstream according to claim 36 wherein said first transmitted datasequence is encrypted.
 40. A method of transmitting a digital televisionstream through a channel comprising the steps of: generating apacketized first data sequence to be compared in a receiver coupled tosaid channel to a second data sequence, said second data sequence beinglocally generated, to provide a channel estimate for adaptivelyadjusting filter coefficients of an adaptive filter such thatundesirable channel effects on a television stream received from saidchannel are filtered from said received television stream; andtransmitting through said channel, a digital television stream includingsaid packetized first data sequence, wherein said first data sequence iscorrupted by noise after passage through said channel to said receiver,said first data sequence corrupted by noise being used to compute anestimate of channel frequency response, wherein the step of comparingcomprises the steps of: computing a Fast Fourier Transform (FFT) of saidfirst data sequence corrupted by noise; computing; a FFT of said seconddata sequence; and dividing the FFT of said first data sequence by theFFT of said second data sequence to provide said estimate of channelfrequency response.
 41. The method of transmitting a digital televisionstream according to claim 40 further comprising the step of determiningsaid channel frequency response using a quotient from said step ofdividing.
 42. The method of transmitting a digital television streamaccording to claim 41 wherein the step of determining said channelfrequency response comprises the step of windowing an Inverse FFT (IFFT)of said quotient of the dividing step.
 43. The method of transmitting adigital television stream according to claim 42 further comprising thestep of estimating noise variance by computing average energy of channelestimation error sequence as a function of said windowed IFFT said firstdata sequence corrupted by noise and said second data sequence.
 44. Themethod of transmitting a digital television stream according to claim 43wherein said step of estimating noise variance comprises the steps of:convolving said windowed IFFT with said second data sequence to generatean estimated noiseless output; subtracting said estimated noiselessoutput from said first data sequence corrupted by noise to generate adifference signal; and computing an average energy estimation from saiddifference signal.
 45. The method of transmitting a digital televisionstream according to claim 44 wherein said channel frequency response andthe estimate of noise variance are used to compute optimum equalizercoefficients for the step of adaptively adjusting filter coefficients.46. A method of transmitting a digital television stream through achannel comprising the steps of: generating a packetized first datasequence to be compared in a receiver coupled to said channel to asecond data sequence, said second data sequence being locally generated,to provide a channel estimate for adaptively adjusting filtercoefficients of an adaptive filter such that undesirable channel effectson a television stream received from said channel are filtered from saidreceived television stream; and transmitting through said channel, adigital television stream including said packetized first data sequence,wherein said first data sequence is transmitted in a dynamic or rollingframe/packet structure.
 47. An apparatus for processing a digitaltelevision signal including a plurality of first data sequencesdistributed over a training interval transmitted in a broadcast channelcomprising: a receiver for receiving said digital television signal,said receiver including; a comparator for comparing said first datasequences to second data sequences, said second data sequences beinglocally generated; and an adaptive equalizer filter including an inputfor receiving said digital television signal, and being in communicationwith said output of said comparator such that filter coefficients ofsaid adaptive filter are adjusted in response to comparator, whereinsaid first data sequences are transmitted in an MPEG Motion PictureExpert Group) channel.
 48. The apparatus for processing a digitaltelevision signal according to claim 47 wherein said first datasequences are corrupted by noise after passage through said channel tosaid receiver.
 49. The apparatus for processing a digital televisionsignal according to claim 48 wherein said first data sequences corruptedby noise are used to compute an estimate of the frequency response ofsaid channel.
 50. The apparatus for processing a digital televisionsignal according to claim 47 wherein said first data sequences areencrypted.
 51. An apparatus for processing a digital television signalincluding first data sequences transmitted in a broadcast channelcomprising: a receiver for receiving said digital television signal,said receiver including: a channel estimator for comparing said firstdata sequences to second data sequences, said second data sequencesbeing locally generated, and for providing an estimate of the impulseresponse of said channel at an output of said channel estimator; and anadaptive equalizer filter including an input for receiving said digitaltelevision signal, and filter taps in communication with said output ofsaid channel estimator such that filter coefficients of said adaptivefilter are adjusted according to said estimate of said impulse responseof said channel, wherein said first data sequences are corrupted bynoise after passage through said channel to said receiver and whereinsaid first data sequences corrupted by noise are used to compute anestimate of the frequency response of said channel, wherein said channelestimator comprises: a first Fast Fourier Transform (FFT) processor forcomputing a FFT of said first data sequences corrupted by noise; agenerator for generating the second data sequences at the receiver; asecond FFT processor for computing a FFT of said second data sequences;and a divider for dividing an output of said first FFT processor by anoutput of said second FFT processor to produce said estimate of channelfrequency response.
 52. The apparatus for processing a digitaltelevision signal according to claim 51 further comprising; an estimatorfor estimating the channel impulse response using a quotient from saiddivider.
 53. The apparatus for processing a digital television signalaccording to claim 52 wherein said estimator includes a processor forwindowing an Inverse FFT (IFFT) of the quotient of the divider.
 54. Theapparatus for processing a digital television signal according to claim53 further comprising a channel estimator configured to provide anestimate of noise variance by computing average energy of channelestimation error sequence as a function of said windowed IFFT, saidfirst data sequences corrupted by noise and said second data sequences.55. The apparatus for processing a digital television signal accordingto claim 54 wherein said noise variance estimator comprises: a convolverfor convolving said windowed IFFT with said second data sequences togenerate an estimated noiseless output; a subtractor for subtractingsaid estimated noiseless output from said first data sequences corruptedby noise to generate a difference signal; and a processor for computingaverage energy estimation from said difference signal.
 56. The apparatusfor processing a digital television signal according to claim 55 whereinsaid channel impulse response and said estimate of noise variance areused to compute optimum equalizer coefficients for adaptively adjustingfilter coefficients.
 57. An apparatus for processing a digitaltelevision signal including first data sequences transmitted in abroadcast channel comprising: a receiver for receiving said digitaltelevision signal, said receiver including; a channel estimator forcomparing said first data sequences to second data sequences, saidsecond data sequences being locally generated, and for providing anestimate of the impulse response of said channel at an output of saidchannel estimator; and an adaptive equalizer filter including an inputfor receiving said digital television signal, and filter taps incommunication with said output of said channel estimator such thatfilter coefficients of said adaptive filter are adjusted according tosaid estimate of said impulse response of said channel, wherein saidfirst data sequences are transmitted in a dynamic or rollingframe/packet structure.
 58. An apparatus for transmitting a digitaltelevision signal through a channel comprising: a generator forgenerating a plurality of first data sequences distributed over atraining interval to be compared in a receiver to at least one seconddata sequence, said second data sequence being locally generated toadjust filter coefficients of an adaptive filter; and a transmitter fortransmitting a digital television signal including said first datasequences through said channel, wherein said first data sequences aretransmitted in an MPEG (Motion Picture Expert Group) channel.
 59. Theapparatus for transmitting a digital television signal according toclaim 58 wherein said first data sequences are corrupted by noise afterpassage through the channel to said receiver.
 60. The apparatus fortransmitting a digital television signal according to claim 59 whereinsaid first data sequences corrupted by noise are used to compute anestimate of the frequency response of said channel.
 61. The apparatusfor transmitting a digital television transmission according to claim 60wherein said channel estimator comprises: a first Fast Fourier Transform(FFT) processor for computing a FFT of said first data sequencescorrupted by noise; a generator for generating said second datasequences at the receiver; a second FFT processor for computing a FFT ofsaid second data sequences; and a divider for dividing an output of saidfirst FFT processor by an output of said second FFT processor to producesaid estimate of channel frequency response.
 62. The apparatus fortransmitting a digital television signal according to claim 61 furthercomprising an estimator for estimating the channel impulse responseusing a quotient from said divider.
 63. The apparatus for transmitting adigital television signal according to claim 62 wherein said estimatorincludes a processor for windowing an Inverse FFT (IFFT) of the quotientthe divider.
 64. The apparatus for transmitting a digital televisionsignal according to claim 63 further comprising a channel estimatorconfigured to provide an estimate of noise variance by computing averageenergy of channel estimation error sequence as a function of saidwindowed IFFT, said first data sequences corrupted by noise and saidsecond data sequences.
 65. The apparatus for transmitting a digitaltelevision signal according to claim 64 wherein said noise varianceestimator comprises: a convolver for convolving said windowed IFFT withsaid second data sequences to generate an estimated noiseless output; asubtractor for subtracting said estimated noiseless output from saidfirst data sequences corrupted by noise to generate a difference signal;and a processor for computing average energy estimation from saiddifference signal.
 66. The apparatus for transmitting a digitaltelevision signal according to claim 64 wherein said channel impulseresponse and said estimate of noise variance are used to compute optimumequalizer coefficients for adaptively adjusting, filter coefficients.67. An apparatus for transmitting a digital television signal through achannel comprising: a generator for generating first data sequences tobe compared in a receiver to second data sequences, said second datasequences being locally generated to provide an estimate of the impulseresponse of said channel for adjusting filter coefficients of anadaptive filter; and a transmitter for transmitting a digital televisionsignal including said first data sequences through said channel, whereinthe digital television signal comprises a high definition television(HDTV) signal and said first data sequences are transmitted in a privatedata stream of an MPEG (Motion Picture Expert Group) channel, whereinsaid first data sequences are encrypted.
 68. An apparatus fortransmitting a digital television signal through a channel comprising: agenerator for generating first data sequences to be compared in areceiver to second data sequences, said second data sequences beinglocally generated to provide an estimate of the impulse response of saidchannel for adjusting filter coefficients of an adaptive filter; and atransmitter for transmitting a digital television signal including saidfirst data sequences through said channel, wherein said first datasequences are transmitted in a dynamic or rolling frame/packetstructure.